The invention relates to a field of open-flow flat-plate solar collectors, and, specifically, to flat-plate solar collectors with wetting the underneath sides of their solar radiation absorbing plates by liquid heat transfer medium.
Such solar flat-plate collectors and heat transferring units are disclosed in some U.S. patents.
U.S. Pat. No. 4,003,365 discloses a structure for collecting solar energy and storing the same as heat in a body of water. An inclined southerly wall is provided with a solar absorption surface protected from convection losses by a transparent membrane overlaying the absorption surface. The absorption surface is also formed as a membrane which is wetted at its underside within the structure by water spray and as solar radiation is absorbed and converted to heat at the membrane, the water at the underside of the membrane is heated. This heated water drops from the membrane and flows to a reservoir.
U.S. Pat. No. 4,085,732 describes a method and apparatus for heating a liquid using solar energy. Through the use of an absorber plate made of a sheet of copper or any other similar conductive metal, the heat from the sun is captured. The front surface of the plate is covered with a dark absorbent coating. The heat absorbed by the copper sheet is conducted to a thin film of liquid such as water which is fed by gravity down the backside of the plate from a supply line disposed adjacent to the top of the backside of the plate. The liquid absorbs the heat from the plate as it flows downward covering the backside of the plate and is discharged at the bottom of the plate into a drain line. The drain line removes the heated liquid from the apparatus.
U.S. Pat. No. 5,460,164 discloses a solar heat collector roof comprises an absorber plate (1) for incident radiation energy and a heat exchange plate placed directly thereunder. The heat exchange plate is corrugated in such a manner that it comprises channels extending down along the roof The heat exchange plate is moreover in direct contact with the absorber plate thereabove. A heat carrying or heat removing liquid medium, e.g. water, flows in the channels. The liquid medium, which removes heat from the heat exchange plate, is supplied at such a rate that the flow rate down along the roof in the channels lies below the rate at which the surface tension of the heat carrying medium is broken, so that a winding (meandering) and travelling flow pattern is imparted thereby to the liquid medium, and up along the channel walls.
U.S. Pat. No. 4,261,337 describes a solar heat collector includes an enclosure having a solar energy transmitting cover member and a solar energy absorbing base member. Within the enclosure, structure is provided, connected to a fluid inlet, for causing a stream or film of fluid to flow along the interior surface of the cover member. The same structure may be used to impart a separate flow of fluid along the energy absorbing base member. The cover member is oriented to prevent the gravitational forces on the fluid stream, flowing along the interior surface of the cover member, from overcoming the adhesion forces between the stream and the interior surface of the cover member. This stream removes condensation from the cover member as it captures heat therefrom. The stream may be created by spraying fluid on the interior surface of the cover member or by a fluid dispensing member having a fluid outlet adjacent the interior surface of the cover. Means for distributing the fluid across the interior surface as same flows therealong may also be provided.
U.S. Pat. No. 4,121,568 teaches a system for collecting heat energy from solar radiation to heat a liquid medium. The system includes a solar radiation collector plate which has its rear face adapted to cause a liquid medium to be in direct thermal contact with the rear face of the plate while flowing over and covering essentially all of the rear face. According to one important aspect of the invention, the rear face is provided with a material having capillary attraction properties to spread a liquid medium across essentially all of the rear face of the plate so that the liquid medium absorbs large amounts of heat energy from the collector plate.
U.S. Pat. No. 4,108,057 teaches a solar water heater is positioned in an inclined manner with an inlet at the upper portion and an outlet at the lower portion so that the water flows downwardly over a series of corrugations extending transversely to the direction of flow of water, the water spilling over each corrugation in turn.
U.S. Pat. No. 4,124,020 describes a solar energy collector has a corrugated, inclined plate exposed to solar rays on a blackened front or top side thereof. A heat-absorbing liquid carrier adheres to an opposite rear or undersurface of the plate in the form of a thin sheet by surface tension, and is directed gravitationally to a collection trough at the bottom edge of the plate.
U.S. Pat. No. 3,995,804 discloses an inclined heat absorptive and conductive panel including downwardly opening inverted V-shaped grooves formed therein extending downwardly from the upper end portion of the panel toward the lower end portion of the panel is provided. In addition, structure is provided for introducing a heat absorptive liquid into the upper end portions of the grooves and second structure is provided for receiving and collecting liquid from the lower ends of the grooves. The cross sectional shape and size of the grooves is such to allow at least substantially all of the liquid introduced into the upper ends thereof to be retained therein by the cohesive and surface tension properties of the liquid during its movement downwardly through the grooves by gravity toward the lower end of the panel. The panel comprises an inclined partition secured across the interior of an upwardly opening housing and a substantially fluid tight cover is secured across the top of the housing above the panel. Further, the structure by which liquid flowing downwardly to the lower end of the panel is collected includes additional structure whereby a partial vacuum is maintained within the housing between the transparent cover and the heat absorptive panel.
U.S. Pat. No. 3,943,911 discloses a solar heat exchanger, which comprises: A. a base and an extended surface thereon for facing frontwardly toward the sun, B. means communicating with said surface to conduct liquid to flow in dispersed condition adjacent said surface, and C. a sheet overlying said surface and spaced therefrom sufficiently closely to cooperate therewith for filming the flowing liquid, said sheet adapted to receive solar radiation for promoting heat transfer to the filmed and flowing liquid
U.S. Pat. No. 3,146,774 describes a solar collector, which is constructed similarly to the solar collector of U.S. Pat. No. 3,943,911.
These patents have a common drawback: in order to ensure complete wetting of the backside of the absorbing plate they would be forced to apply relatively high flow rates (200 kg/mh or more) of water or aqueous solution supplied into their distributing pipes. This value of the required flow rate is presented, for example, in the book: S. S. Kutateladze HANDBOOK OF HYDRODYNAMIC PRESSURE DROPS AND HEAT TRANSFER, Energoatomizdat, Moscow 1990, p. 178 (in Russian).
On the other hand, flow rate of 40 kg/mh or less is sufficient for a common 1.8-square-meter flat plate solar collector. For the flat plate solar collectors, which are intended to concentrate a diluted solution of liquid desiccant (as, for example, aqueous solutions of LiC1 or CaCl.sub.2) this flow rate can be estimated as 10-15 l/hm.
In such a way, the aforementioned common solar collectors require usage of pumps with relatively high power; it leads to additional expenses for equipment and electric energy.
Only U.S. Pat. No. 3,995,804 has not this drawback and can operate with low flow rates. However, this patent does not give solution of anticorrosive polymer coating of the backside of the solar radiation absorbing plate (such polymer coating can fulfill the inverted V-shaped grooves described in this patent). In addition, U.S. Pat. No. 3,995,804 does not solve a problem of a relatively small general surface of rivulets flowing on the backside of the solar radiation absorbing panel.
Secondly, it is known that oxygen entering into an open loop hydraulic solar system will cause rust in any iron or steel component. Such systems should have copper, bronze, brass, stainless steel, plastic, rubber components in the plumbing loop.
Therefore, if the solar radiation absorbing plate is fabricated from a common carbon steel sheet, its backside to be coated with a layer of thermo-stable polymer anticorrosive material.